Overlay sheet for woody board



This invention relates to the bonding of an adhesively coated sheet or layer of material to a base as required in the manufacture of certain composite articles.

More specifically, this invention relates to an improved process for securing a surfacing material, such as a fibrous sheet, specially treated paper, etc., to panels or boards of wood or plywood or other lignocellulosic boards, and, in particular, for securing the so-called overlay or resin-treated paper to the surface of plywood.

Since this invention at present has as its most important use the bonding of overlay paper to plywood, it will be described as employed in this connection.

This application is a continuation-inpart of our abandoned application, Serial No. 375,848, filed under date of August 21, 1953, entitled Thermosetting Adhesive and Method of Making the Same.

It is customary in the bonding of overlay paper to the surface of plywood to use a thermosetting resin as the adhesive, and primarily a phenol-aldehyde type of resin is employed. The resin adhesive is generally applied to the paper in the form of a continuous film coating on one side of the paper, the coating operation being usually performed by the paper manufacturer through the medium of a conventional coating machine. Then the paper with the dried coating is shipped to the plywood mills in ready condition for bonding with the plywood.

The mounting of the overlay paper on the plywood panels is generally done with commercial hot presses which have a number of openings in each press (for example, from ten to twenty), so that a number of panels can be completed simultaneously. At the present time such presses are generally hand loaded, the plywood panels with their overlay" paper being inserted in place in the press one at a time.

Failures in securing the desired complete and permanent bonding of the overlay paper to the face of the plywood with the above procedure have been reported quite frequently, and, in our efforts to discover the reasons for such failures and to find a remedy for the same, we have encountered three main problems.

One of these problems arises from the fact that since the coating of the overlay paper with the resin adhesive is performed by the paper manufacturer and the coated paper is shipped to the plywood mills in ready-touse form, for subsequent employment, the coated paper may be held in storage for considerable time before its actual bonding to the plywood takes place. Consequently, the adhesive coating should be capable of adequate life in storage in dry form. It is known that a lowmolecular weight phenol-aldehyde resin in general will survive a long storage period better than a high-molecular weight phenol-aldehyde resin. However, a lowmolecular weight resin in itself, we found, would not solve our problem, mainly for the reasons to be presently explained.

A second and related difiiculty we found was caused by the procedure followed during the hot pressing of the coated overlay paper to the plywood base. Since, as previously mentioned, the commercial hot presses which are employed generally are arranged for pressing a numher of panels at the same time and since the presses are hand loaded, several minutes may elapse between the loading of the first panel in such a press and the loading of the last panel before the actual pressing of the panels in the press occurs. During the time the panels are in the press prior to the pressing operation the adhesive is exposed to an elevated temperature without any pressure being applied. This subjection of the resin of the adhesive to an elevated temperature is known as a pre-cure interval and serves to cure the resin partially before the application of the pressure and the resulting final curing. The Pre-cure interval thus is prolonged for the panels inserted in the press first. A low-molecular weight phenol-aldehyde resin is known to have a longer precure time than a high-molecular weight phenol-aldehyde resin, but, as presently explained, a low-molecular weight phenol-aldehyde resin in itself is not satisfactory.

While a low-molecular weight phenol-aldehyde resin does have a longer storage life and a longer pre-cure time, the third problem encountered was presented by the fact that a low-molecular weight phenol-aldehyde resin would produce a weaker and less satisfactory bond between the paper and the plywood due to the tendency of such resin to penetrate excessively into the paper, thus reducing the strength of the bond film beween the paper and the plywood or base. Furthermore, a low-molecular weight phenol-aldehyde resin is unsatisfactory when applied as a coating on a sheet of paper because of its tendency to stick or block.

The primary object of our present invention, accordingly, is to provide an improved process for surfacing a woody board with an overlay sheet by employing a thermosetting adhesive resin composition which will not penetrate excessively into the sheet during the coating operation but will have a good hold-out.

With thermosetting adhesive compositions of the phenol-aldehyde type which have previously been developed for bonding overlay paper to plywood it has been found that less unsatisfactory results can be obtained by adding a humidifying step in order to provide sufficient moisture to plasticize the resin of the adhesive coating during the bonding operation.

An additional object of our invention is to provide a composite woody panel overlaid with a fibrous sheet bonded to the board by a special water-insensitive, thermosetting resin adhesive which will dry quickly to a nontacky film, when first applied as a coating to the sheet prior to the bonding, which will be capable of long storage life, and which will have a reasonably long precure time, immediately preceding the hot pressing bonding of the sheet to the board, but which will not require and further humidifying or additional plasticizing treatment prior to or during the hot pressing, and which will insure improved bonding of the overlay paper under I the customary hot pressing bonding procedure.

After considerable experimenting we have succeeded in attaining these objects satisfactorily with the employment of an improved adhesive composition having a thermosetting phenol-aldehyde resin and a thermoplastic polyvinyl acetate resin as the essential elements. By combining these two types of resins in the form and with the proportions hereinafter described, the resulting adhesive has proved to be very satisfactory in the tests in which it has been employed with overlay paper on plywood panels. Such adhesive has been found not only to have a relatively long storage life and an adequate and increased pre-cure time, but also to dry quickly to a tackfree film in the coating operation, with only slight penetration into the sheet, and, after the bonding operation, to exhibit increased boil resistance. Further we have found that the presence of the polyvinyl acetate in the adhesive composition aids in promoting a more even wetting of the sheet when applying the adhesive.

For the thermosetting phenol-aldehyde resin to serve as the main ingredient in the special adhesive composition, we have found that an alkali-catalyzed, water-soluble, alkaline condensation product of phenol and formaldehyde, manufactured and sold by the American-Marietta -Company under the trade name Amres 1400, is very desirable. This resin is characterized by a relatively long setting period, which may be determined by the hot-plate stroke cure test. This test consists of spreading 0.1 milliliter of the resin on a small glass plate, placing the glass plate on a hot plate heated to 150 C., and stroking the resin with a spatula or a stirring rod until the resin completely sets or cures. The time required for the resin Amres 1400 to cure under foregoing test conditions is 3'4 seconds, whereas prior art phenol-aldehyde type thermosetting resins used in bonding woody products required only about 10 seconds. Other characteristic properties of Amres 1400 resin are: specific gravity 1.20:0.01, viscosity 550-880 centipoises, and pH of 93:01. This resin has a low molar ratio of formaldehyde to phenol, and its dilutability with water is about 3.5 parts water to 1 part resin at a temperature of 25 C. We have found that other thermosetting phenol-aldehyde resins having similar properties to Amres 1400 and which are compatible or dispersible with the'polyvinyl acetate resin may be also used. One of these is a resin supplied by the Monsanto Chemical Company under the designation G 37-106, which we have found to be almost as satisfactory for our purpose as the above-mentioned Amres 1400 resin of the American Marietta Company.

For the thermoplastic resin constituent in this special adhesive composition we use a water emulsion of polyvinyl acetate which is produced by E. I. du Pont de Nemours and Company, Inc., under the trade name of Elvacet 81-900. The characteristic properties of this emulsion are: specific gravity at 25 C. 1.11, viscosity 800-1000 centipoises, pH at 25 C. 4-6, and monomeric vinyl acetate content less than 1%. Other polyvinyl acetate emulsions which we have used satisfactorily are Polyco 117 and Polyco'345, which are manufactured by the American Polymer Corporation, and Gelva S-55, which is manufactured by the Shawinigan Chemical Company. Furthermore, other thermoplastic resins in form of dispersions, or emulsions, such as plasticized polystyrenes or polyamidegmay be usedin lieu of polyvinyl acetate. We found that these compounds have the property of improving p're-c1'1re when mixed with a phenol-aldehyde resin. Examples of satisfactory polystyrenes are BKS-92 manufactured by Bakelite Corporation, the characteristics of which are specific gravity at 25 C. 1.03, viscosity =1500 cps., pH 9.0, or Lustrex 620 emulsion produced by Monsanto Chemical Company the characteristics of which are specific gravity at 25 f C., 1.04, viscosity from 5-15 cps., pH from 8 to 9. A satisfactory plasticized polyamide resin cationic water dispersion is B-ZOO, the characteristics of which are acid number 3-5, specific gravity at25 C. 1.02, viscosity from 50 to 130 cps. and pH from 4.9-5.2 and a satisfactory.non-plasticized polyamide resin cationic water dispersion is A-000, the characteristics of which are acid number 3-5, specific gravity at 25 C. 1.01, viscosity from 40 to 90 cps., pH from 4.9-5.2 and average molecular weight of resin 3,000-6,500, both sold by General Mills Incorporated.

The coating of a fibrous sheet with such a special adhesive composition comprising essentially a major proportion of phenol-aldehyde and a minor proportion of polyvinyl acetate resins will produce a somewhat slippery surface on the sheet. This may cause some inconvenience in the handling or shipping of the coated sheet. This condition is satisfactorily avoided by incorporating an inert filler in the adhesive composition, preferably such an inert filler-as nut shell flour, wood flour, or bark flour. As a preferred example, we have found it very satisfactory to use for this purpose a walnut shell flour which is manufactured by Agrashell, Incorporated, under the trade name Glufil WF-7. The addition of such inert filler to the adhesive composition produces a roughness to the coated surface of the paper and thus improves and facilitates the handling of coated paper during plyood pressing operations.

As an optional and auxiliary ingredient in the special adhesive composition, we have found that the addition of ammonium hydroxide or other organic bases, such as amines, to the mixture will improve the dispersion of the ingredients and also increase somewhat the storage life of the adhesive composition before it is applied to the paper as a coating. For this purpose we prefer to use a commercially available 28% aqueous ammonium hydroxide solution.

Water may be added to the mixture primarily for the purpose of adjusting the viscosity to the degree desired.

The relative proportions of the above mentioned ingredients may be varied within limits to suit conditions. In the following table we give what we regard as the operative range within which the relative proportions may be varied and also give a narrower preferred range.

Operative Preferred Ingredient Range Range (parts by (parts by Weight) weight) Phenol-Aldehyde Resin (about 64% solids) 1, 000 1,000 Polyvinyl Acetate Resin (about 55% solids) 50-600 -200 Inert Filler (dry powder) 50-150 60-100 28% Aqueous Ammonium Hydroxide. 0.2-1.0 0. 3-0. 6 Water 0-700 250-500 (sufilcieut to attain desired r iscosity) Thus, as a basis, in each case we have taken phenolaldehyde resin in the relative amount of 1000 parts by weight, using the phenol-aldehyde resin in the form of an aqueous solution containing about 64% solids; the other essential ingredient, as previously mentioned, being a polyvinyl acetate resin emulsion. The inert filler, ammonium hydroxide, or water may be incorporated optionally into the mixture in any combination.

Thefollowing are illustrative examples of the special adhesive composition formed with the particular ingredients mentioned and in specific relative proportions which we have found to be very satisfactory.

Example 1 Parts by weight Phenol-aldehyde resin 1000.0 Polyvinyl acetate resin 145.0 Water 375.0

The phenol-aldehyde resin used was the Amres 1400 manufactured by the American Marietta Company, previously mentioned, in the form of an aqueous solution containing about 64% solids; and the polyvinyl acetate resin was the Elvacet 81-900 manufactured by the E. I. du Pont de Nemours and Company, in the form of an aqueous emulsion having approximately a 55% solids content.

The polyvinyl acetate resin was added slowly, with stirring, to the phenol-aldehyde resin at room temperature and the mixture was agitated for 5 minutes, at the end of which time the blending of the phenol-aldehyde resin and the polyvinyl acetate resin was substantially complete. Water was then added, with stirring. The final mixture had a Brookfield viscosity of 670 centipoises.

Example 2 Parts by weight The phenol-aldehyde resin and the polyvinyl acetate resin used in this case were the same as those mentioned in Example 1. The walnut shell flour was Glufil WF-7, sold by Agr-ashell, Incorporated.

An aqueous slurry of walnut shell flour was made by first adding the walnut shell flour to 190 parts by weight of water and stirring until the slurry was free of lumps. This slurry was added slowly to the phenol-aldehyde resin and agitated for 10 minutes. The polyvinyl acetate resin was then mixed in, with stirring, and agitated for minutes, at the end of which time the blending of the above ingredients was substantially complete. Finally the remaining 185 parts of water were added, with stirring. The final composition had a viscosity of 970 centipoises.

Example 3 Parts by weight Phenol-aldehyde resin 1000.0

Polyvinyl acetate resin 145.0 28% ammonium hydroxide 1 0.4 Water 375.0

The phenol aldehyde resin and the polyvinyl acetate resin used were those mentioned under Example 1. The ammonium hydroxide used was a commercial 28% aqueous solution of ammonia.

The ammonium hydroxide was added to parts of water. This solution was mixed into the phenol-aldehyde resin under agitation. The polyvinyl acetate resin was then added slowly, with stirring, and agitated for 5 minutes, at the end of which time the blending of the above ingredients was substantially complete. The remaining 365 parts of water were then mixed in with agitation. The Brookfield viscosity of the final mixture was 680 centipoises.

Example 4 Parts by weight Phenol-aldehyde resin 1000.0

Polyvinyl acetate resin 145.0 Walnut shell flour (dry powder) 80.0 28% ammonium hydroxide 0.4 Water 375.0

The phenol-aldehyde resin and the polyvinyl acetate resin were the same as mentioned in Example 1; the walnut shell flour was the same as mentioned in Example 2; and the ammonium hydroxide was the same as mentioned in Example 3.

A walnut shell flour slurry was first prepared by adding the flour to 165 parts of water containing the 0.4 part of 28% ammonium hydroxide, and the slurry was stirred until free of lumps. The slurry was then added slowly to the phenol-aldehyde resin with continuous stirring and agitated for 10 minutes. The polyvinyl acetate resin emulsion was then mixed in slowly, with stirring, and agitated for 5 minutes, at the end of which time the blending of the above ingredients was substantially complete. The remaining 210 parts of water were then added, with stirring. The viscosity of the final mixture was 925 centipoises.

In all the above examples the compounding took place at room temperature. All the viscosity measurements were taken at 25 C.

Any of the hereinabove described adhesive compositions may be applied to a fibrous overlay sheet as a continuous film coating in the amount of between 5 and 50 pounds (solids basis) per ream of overlay, preferably between 10 and 30 pounds per ream.

We made extensive tests in employing the adhesive of Example 4 for the bonding of overlay paper to plywood panels. The adhesive was applied to one -face of a phenolic resin-containing paper as a coating in the amount of 25 lbs. adhesive (solids basis) per ream (3000 square feet), the adhesive being applied at room temperature and dried in a steam-heated oven to a non-tacky state. The bonding of the overlay paper to the plywood was performed in a heated press with :a pressure of 200 lbs. per square inch and at a temperature of about 285 F., during a period of 7 minutes. The resulting product, namely the plywood with the overlay paper bonded thereto by the above desrcibed adhesive composition, passed 6 cycles of the National Bureau of Standards weathering test entitled Accelerated Aging of Fiber Building Boards and described in Building Materials and Structures, Report BMS 4, National Bureau of Standards, US. Department of Commerce (1938). The bond produced by the adhesive proved to be very strong, water proof, boilproof, alkaliand acidresistant.

The dry adhesive coating film has a preacure time of about 3 minutes, which is longer than that of other phenol-aldehyde resin adhesives heretofore used for the same purpose. Overlay paper coated with this adhesive can be stored for a period of one year under normal temperature and humidity conditions, during which time the adhesive coating will have no apparent loss of any of its originally imparted properties. The adhesive composition has a better flow under curing conditions than the unmodified phenol-aldehyde adhesives, thereby producing a more intimate and complete bonding of the overlay sheets to rough plywood surfaces.

Accordingly, it is apparent that by the present invention we have provided an improved process for the manufacture of overlaid woody boards which process includes the use of fibrous cellulosic sheets coated with a special thermosetting adhesive. Through interaction of this adhesive, the process results in the formation of composite boards characterized by a durable and strong bond between the overlay sheet and the board.

We claim:

An improved fibrous overlay sheet for use in surfacing a woody board, said sheet carrying a coating film of adhesive substantially retained on the face of the sheet and comprising, as the main ingredients, a major proportion of a thermosetting phenol formaldehyde resin and a minor proportion of thermoplastic polyvinyl acetate resin, said coating film characterized by being water-insensitive and dry but being heat-activatable and by a relatively long pre-cure time of three minutes when exposed to an elevated temperature immediately prior to hot pressing of the sheet to the wood board, whereby the adhesive will remain as a water-insensitive coating substantially on the face of the sheet until the bonding of the sheet, and whereby the coating will remain non-tacky until the heat activation of the adhesive occurs in the bonding of the sheet.

References Cited in the file of this patent UNITED STATES PATENTS 2,343,740 Birmingham Mar. 7, 1944 2,474,292 Weidner et a1 June 28, 1949 2,564,291 Wolf Aug. 14, 1951 2,613,167 Cone Oct. 7, 1952 2,658,840 Perry Nov. 10, 1953 2,675,335 Rankin et al Apr. 13, 1954 2,748,046 Works et al May 29, 1956 2, ,4 o k a -,T J 2 8 

